In recent years the screen printing industry has undergone enormous growth, due primarily to the great popular appeal of shirts, caps, jackets, and other apparel bearing printed designs and logos. The screen printed apparel business continues to expand, providing opportunities for thousands of print shops across the country.
Generally speaking, most printed apparel displays a multi-color image, and thus requires multiple printing steps employing color separation images printed in accurate alignment. There are two broad categories of printing machines which are capable of multiple screen printing; manually operated machines, which typically exhibit low output rates, and fully mechanized, automatic printing machines which typically have very high printing rates. However, automatic screen printing machines require a very large capital investment, and also require extensive set-up time and maintenance. In the apparel market, many items are printed in small lots, and thus are not suitable for automatic machine printing. When these small lots are printed on a manual machine, the slow output rate and higher labor factor causes the overall cost to increase.
A typical manually operated multiple screen printing machine known in the prior art provides a plurality of screens bearing the color separation images, each of the screens supported on an individual screen arm. All of the screen arms extend from a rotatable table, and are spaced in supposedly equal angular fashion about the axis of rotation. A plurality of printing platens are disposed below the screens, the platens also being supported by a rotating table having a common axis with the screen table. The platens are also spaced in supposedly equal angular increments about the rotational axis.
With one worker stationed adjacent to each of the platens, it should be possible for all of the workers to pull down the screens simultaneously, ink and print the images, and then lift the screens. The workers should then be able to rotate the platens to the next printing station, and reiterate the printing process. Several manual printing machines currently being sold claim to be able to carry out this high output form of manual printing.
Unfortunately, this high speed mode of printing requires that all of the screens and all of the platens are spaced in perfect equal angular fashion. If one of the platens or one of the screens arms are spaced out of tolerance, even by a fraction of a degree, that screen or platen will not register with its respective platen or screen during the printing operation. Although the two components may be forced manually into registration, the common table mounting of the platens and the screens will cause all of the coupled components to tend to shift angularly as the out of tolerance component is forced into alignment. The result is a printed image in which the color separations are not in accurate alignment, and the article is not suitable for sale.
It is an empirical fact that equal angular spacing of multiple components on a large radius assembly is difficult to achieve and sustain. Slight angular misalignments in multiple screen printing machines are almost unavoidable. Thus, in the prior art many manually operated screen printing machines which claim to provide high speed, simultaneous printing are actually operated in serial printing fashion: each screen is pulled down and printed individually and separately from the others. The obvious consequence is that the printing output rate is far lower than advertised by the printing machine manufacturer.